RF power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as a cascade gain. Ideally, the input-to-output transfer of each stage is linear; a perfect replica of the input signal, increased in amplitude, appears at the amplifier output. In reality, however, all RF power amplifiers have a degree of non-linearity in their transfer characteristic. This non-linearity results in the distortion of the output signal so that it is no longer a perfect replica of the input. This distortion produces spurious signal components known as intermodulation (IM) products. Intermodulation products are undesirable because the cause interference cross-talk, and other deleterious effects on the performance of a system employing RF power amplifiers. Accordingly, the prior art reflects various methods and devices designed to reduce the distortion produced during RF power amplifier operation. Two methods commonly suggested are predistortion and feed forward.
Predistortion utilizes an auxiliary distortion source which produces an auxiliary distortion signal similar to the distortion generated by a power amplifier. The auxiliary distortion signal is added to the power amplifier input in the correct gain and phase to promote cancellation of the distortion at the output of the power amplifier. This method requires matching the distortion characteristics of two dissimilar sources and hence limits the amount of correction which can be obtained.
Feed forward amplifier circuits are known in the art. Feed forward amplifier circuits utilize a feed forward technique in which a sample of the distortion generated in a power amplifier is coupled off, isolated, amplified, and recombined 180 degrees out of phase, in order to cancel the remaining distortion in the output signal. In general, feed forward amplifier circuits separate out distortion and intermodulation components generated by a power amplifier in order to create an error signal. The error signal is then added to the power amplifier's output with a gain, a phase shift, and delay. The gain, phase shift, and delay are adjusted for maximum cancellation of the intermodulation and distortion generated by the power amplifier, in an attempt to produce an amplified output signal which is free of distortion. In essence, the error components which are created by the power amplifier are subsequently subtracted out of the amplified signal. The amount of distortion reduction available using feed forward technology is limited by the accuracy of gain and phase adjustments of the error signal. Prior art feed forward amplifiers have attempted to increase the accuracy of the gain and phase adjustments by injecting a test signal, or pilot, into the main signal. The test signal is then utilized to adjust the gain and phase of the error cancellation signal. The problem with utilizing a pilot tone for controlling the gain and phase of the error signal is that the addition of a pilot tone generator adds appreciable cost, board space, and isolation requirements to any feed forward amplifier.
Typical prior art feed forward amplifiers implement distortion minimization circuits which provide continuous, and substantially accurate, gain and phase adjustments. Feed forward amplifier accuracy over a wide range of frequencies and amplitudes results from utilizing both carrier and intermodulation cancellation, controlled by the detection of the total power of the intermodulation distortion via an intermodulation controller, rather than by an injected pilot tone. Although this circuitry provides a substantial improvement over other prior art feed forward circuits, it utilizes bulky RF hardware in its intermodulation controller. Such bulky RF hardware may include delay lines and couplers commonly associated with analog applications. In applications which require digital implementation due to smaller space availability and tighter specification tolerance, utilization of a digital signal processor (DSP) is desirable. In addition, an intermodulation controller implemented via a DSP would also result in a lower part count and hence reduced cost. Also, utilizing a digitally implemented IM controller in place of the prior art analog implemented IM controller affords more accurate control of the gain and phase adjustments to the error signal due to improved carrier cancellation and a more accurate estimate of the total power of the intermodulation distortion.
Therefore a need exists for a method and apparatus to digitally control the gain and phase of an error signal in a feed forward amplifier circuit which overcomes the prior art problems.